In general, a solar cell is a device that converts light energy of the sun into electric energy. That is, the solar cell is a device that produces electricity using sunlight which is an infinite energy source. A silicon solar cell, one kind of such solar cell, is already widely used in daily life. Recently, a dye-sensitized solar cell is attracting attention as a next-generation solar cell. The dye-sensitized solar cell is a photo-electrochemical solar cell having higher efficiency and much lower manufacturing cost per unit, as compared to the conventional silicon solar cell. Thus, the dye-sensitized solar cell is expected to replace the conventional silicon solar cell.
One representative example of such a dye-sensitized solar cell is one developed by Michael Gratzel's research team of EPFL (Ecole Polytechnique Federale de Lausanne, Switzerland) in 1991 (see, for reference, U.S. Pat. No. 5,350,644, “Photovoltaic Cells”). In the structural aspect, one of two electrodes of the dye-sensitized solar cell is a photoelectrode including a transparent conductive substrate forming thereon a semiconductor layer on which a photosensitive dye is adsorbed. Electrolyte is filled in a space between the two electrodes.
A basic principle of the operation of the dye-sensitized solar cell is as follows. As solar energy is absorbed by the photosensitive dye adsorbed onto the semiconductor layer of the one electrode, and thus photoelectrons are generated. The photoelectrons are transferred to the transparent conductive substrate on which a transparent electrode is formed by conducting through the semiconductor. The dye oxidized as a result of losing the electrons is reduced by redox pairs included in the electrolyte. Meanwhile, electrons that reach the counter electrode on the opposite side through an external electric line reduce the redox pairs of the oxidized electrolyte. In this way, the solar cell is operated.
Meanwhile, as compared to a conventional solar cell, the dye-sensitized solar cell has many interfaces such as a semiconductor/dye interface, a semiconductor/electrolyte interface, a semiconductor/transparent electrode interface, an electrolyte/counter electrode interface, and so on. Understanding and controlling physical and chemical reactions that take place at these interfaces is the key issue of the dye-sensitized solar cell technique. Further, energy conversion efficiency is in proportion to the amount of the photoelectrons generated by the absorption of the solar energy. In order to generate a great amount of photoelectrons, it is required to fabricate a photoelectrode including a structure capable of increasing an adsorption amount of dye molecules.
In general, a ruthenium (Ru) metal complex has been widely used as a dye for the dye-sensitized solar cell. For example, Korean Registered Patent No. 10-0578798 (“Dye-sensitized solar cell and method of manufacturing the same”) describes a dye-sensitized solar cell including ruthenium as a photosensitive dye. However, the ruthenium metal complex has drawbacks in that it is very expensive and difficult to purification. Further, it takes a long time ranging from a minimum of 2 hours to a maximum of 24 hours to adsorb an organic dye containing ruthenium metal onto the semiconductor layer. As a result, it takes a long time to complete the manufacture of the dye-sensitized solar cell. Further, since the organic dye containing the ruthenium metal cannot absorb light having a wavelength equal to or lager than about 800 nm, it is impossible to utilize light in a long wavelength range. Thus, there exists a limit in improving photoelectric current.